1. Field of the Invention
The present invention relates to a quartz crystal oscillator having a quartz crystal unit and an oscillation circuit which uses the crystal unit for supplying an oscillation frequency signal, and more particularly to a surface-mount crystal oscillator which is amenable to miniaturization and in which a receptacle which contains an IC (Integrated Circuit) chip is bonded to the rear surface of a crystal unit.
2. Description of the Related Art
Surface-mount crystal oscillators, and in particular, surface-mount temperature-compensated crystal oscillators (TCXOs) feature light weight, compact size, and an oscillation frequency having superior stability, and these devices are therefore widely used in communication equipment used in a mobile environment, such as portable telephones. One type of surface-mount crystal oscillator is a bonded type surface-mount crystal oscillator in which a mounting substrate is installed as a receptacle on the rear surface of a crystal unit. An IC chip that together with the crystal unit makes up the oscillation circuit, and other electronic components are mounted on the mounting substrate.
FIG. 1 shows an example of the construction of this type of surface-mount crystal oscillator of the prior art. This surface-mount crystal oscillator is provided with crystal unit 1 and mounting substrate 2. Crystal unit 1 is constructed by accommodating quartz crystal blank 3 within substantially rectangular parallelepiped casing 4 and covering the opening of casing 4 by metal cover 5. Casing 4 has a shape in which a concavity is formed in one of its principal surfaces.
Crystal blank 3 is, for example, a substantially rectangular AT-cut quartz crystal blank, and although not shown here, excitation electrodes are formed opposite each other on the two principal surfaces of the crystal blank. In addition, extension electrodes are formed on both sides of one end of crystal blank 3 to extend from the excitation electrodes. Both sides of one end of crystal blank 3 from which the extension electrodes extend are secured to terminal electrodes on the bottom surface of the concavity of casing 4 by means of conductive adhesive 7. Crystal blank 3 is thus held horizontally and connected electrically and mechanically to casing 4.
A pair of connection terminals 6a are provided at both ends of one diagonal of the outer side of the bottom surface of casing 4. These connection terminals 6a are each connected to respective terminal electrodes which connect to crystal blank 3 by way of conductive paths formed in casing 4. In addition, ground terminals 6b are provided at both ends of the other diagonal of the outer side of the bottom surface of casing 4. Ground terminals 6b are electrically connected to metal cover 5.
Mounting substrate 2 is composed of ceramic, has a substantially rectangular planar shape, and has a concavity or recess formed in one of its principal surfaces. More specifically, mounting substrate 2 is made up by substantially rectangular planar bottom wall 8 and sidewalls 9 which are formed in a substantially rectangular frame shape. Sidewalls 9 are provided around the entire outer perimeter of mounting substrate 2, thereby defining the concavity. This concavity accommodates IC chip 10, in which an oscillation circuit connected to crystal unit 1 is integrated, and two chip capacitors 11. IC chip 10 may incorporate a temperature compensation mechanism which compensates the temperature dependency of the oscillation frequency of the crystal oscillator. As chip capacitors 11, a variety of large-capacitance capacitors which are difficult to integrate inside IC chip 10 may be arranged, including bypass capacitors between the power supply and ground, capacitors for coupling with a succeeding stage, or CR filter capacitors for suppressing noise which is produced by the temperature compensation mechanism.
Connection terminals 12a corresponding to connection terminals 6a of crystal unit 1, and ground terminals 12b corresponding to ground terminals 6b of crystal unit 1 are provided on the upper surfaces of sidewalls 9 which surround the concavity on mounting substrate 2. In addition, mounting terminals 14 including terminals such as a power supply terminal, ground terminal, output terminal, and AFC (Auto Frequency Control) terminal are provided on the principal surface of mounting substrate 2 on which the concavity is not formed. These mounting terminals 14 electrically connect to corresponding terminals on IC chip 10. IC chip 10 is also electrically connected to connection terminals 12a and ground terminals 12b. Finally, when this crystal oscillator is a temperature-compensated crystal oscillator, write terminals are provided on the outer surfaces of sidewalls 9 for writing temperature compensation data to the temperature compensation mechanism inside IC chip 10.
This crystal oscillator is assembled by using, for example, solder to connect connection terminals 6a and ground terminals 6b of crystal unit 1 to connection terminals 12a and ground terminals 12b of mounting substrate 2. Crystal unit 1 and mounting substrate 2 can therefore be fabricated in parallel, meaning that this type of surface-mount crystal oscillator can be manufactured with high efficiency. Further, since the electrical characteristics of crystal unit 1 can be checked before joining crystal unit 1 with mounting substrate 2 which is equipped with IC chip 10, this crystal oscillator offers the additional advantage of preventing the waste of expensive IC chips 10 when crystal units 1 having inadequate characteristics occur.
When fabricating mounting substrate 2, a ceramic substrate is first prepared in sheet form having a size which corresponds to a multiplicity of mounting substrates 2. Concavities corresponding to each individual mounting substrate have been formed in the ceramic sheet. A multiplicity of mounting substrates 2 can therefore be fabricated simultaneously by splitting this sheet ceramic substrate along break lines which extend in the horizontal and vertical directions.
With the further development of miniaturization of surface-mount crystal oscillators in recent years, however, mounting substrates having planar dimensions of, for example, 3.2 mm×2.5 mm have come into use. When the planar dimensions become this small, the problem arises that the concavity of mounting substrate 2 in the above-described surface-mount crystal oscillator can accommodate no more than IC chip 10 and can no longer accommodate chip capacitors 11.
As a countermeasure, portions of sidewalls 9 of mounting substrate 2 can be removed and chip capacitors 11 then arranged in these portions, as shown in FIG. 2. In other words, sidewalls 9 are not formed in the central areas of each of a pair of mutually opposed edges of mounting substrate 2, whereby the surface of bottom wall 8 is exposed and chip capacitors 11 are arranged in these areas. Since portions of sidewalls 9 are not provided, the concavity of mounting substrate 2 is open at the two longitudinal ends of mounting substrate 2.
The adoption of mounting substrate 2 in which the concavity is open in the two longitudinal end directions, however, raises problems in fabrication of mounting substrate 2. FIGS. 3A and 3B are figures for explaining the division of the sheet ceramic substrate into individual mounting substrates. As previously described, mounting substrates 2 are fabricated by dividing sheet ceramic substrate 15 corresponding to a multiplicity of mounting substrates along break lines A—A and B—B which are extend in two-dimensional directions. FIG. 3B shows a sectional view of sheet ceramic substrate 15 along break line A—A. When dividing ceramic substrate 15, a v-shaped groove is formed in the upper surface of sidewalls 9 along the break lines to facilitate splitting. In addition, a v-shaped groove is also formed on the rear surface of sheet ceramic substrate 15 which is opposite the v-shaped groove in the upper surface of sidewalls 9. However, this v-shaped groove cannot be formed at positions in which sidewalls 9 have been removed, i.e., on the exposed surface of bottom wall 8, because the thickness of the ceramic substrate is reduced in these areas. When v-shaped grooves are formed in this way, splitting can be easily achieved along break lines B—B along which the thickness of ceramic substrate 15 is uniform. Along break lines A—A, however, the variation in the thickness of ceramic substrate 15 and the lack of V-shaped grooves at the open-end portions of sidewalls 9 causes the stress to disperse to the thin portions of the substrate. This results in such problems as uneven breaks in sheet ceramic substrate 15 along break lines A—A and the occurrence of damaged mounting substrates 2, whereby productivity drops and yield is extremely degraded.